1. Field of the Invention
The present invention relates to a lithium polymer battery and a method for manufacturing the same. In particular, the present invention relates to a lithium polymer battery that is capable of forming a simple external sheath while securing reinforcement members and a method for manufacturing the same.
2. Description of the Prior Art
As generally known in the art, a lithium polymer battery has a separator that is positioned in between a positive electrode plate and a negative electrode plate to serve as a medium for ion conductance and as a medium for separating the electrodes in the lithium ion battery. The separator may comprise a gel-type polymer electrolyte, which is prepared by impregnating a polymer with an electrolyte to improve ion conductivity.
In addition to improved ion conductivity, the gel-type polymer electrolyte has the advantages of excellent bonding to electrodes, mechanical properties, and a simple manufacturing process. A PVDF-based electrolyte (available from Bellcore Company) is a gel-type polymer electrolyte that is prepared by mixing a copolymer of vinylidene fluoride (VDF) and hexafluoroethylene (HFE), a plasticizer, and an inorganic additive to form a film, impregnating it with an electrolyte, and causing it to gel.
The characteristics of lithium polymer batteries and lithium ion batteries will now be compared briefly.
Lithium polymer batteries may have a plate structure and do not need to be wound as in the case of lithium ion batteries. Therefore, the electrode assembly comprising a number of positive electrode, a negative electrode, and a separator plates can be laminated and may be suitable for use in a rectangular structure. Of course, lithium polymer batteries can have a wound electrode assembly.
In addition, the electrolyte in a lithium polymer battery is injected into a completely integrated electrode assembly which prevents leakage of the electrolyte.
The plate structure of an electrode assembly of a lithium polymer battery does not require the application of pressure to form a rectangular shape. Therefore, a thin flexible pouch may be used as the battery sheath instead of a hard rectangular or cylindrical can.
When a flexible pouch is used as the sheath of a lithium polymer battery, the thickness of the resulting battery is substantially reduced compared with using a can as the sheath. In addition, more electrode assemblies can be contained in the same volume. This increases the power storage capacity of the battery. The battery can be easily manufactured in any desired shape due to the flexible sheath and can easily be mounted on a variety of external devices.
However, such a pouch-type lithium polymer battery has various problems resulting from its weak protective cover. For example, if the pouch-type sheath of a conventional lithium polymer battery is pierced by a sharp object such as a needle or a nail, a puncture is easily formed. If the sheath is bitten by a pet, it is easily torn off. Furthermore, when a sharp object penetrates the sheath and contacts the internal electrode assembly, the internal positive electrode plate and the negative electrode plate may be short-circuited. The battery may then catch fire or explode.
Conventional lithium polymer batteries do not radiate heat well and this may shorten the lifespan of the battery. The pouch-type sheath basically has an insulation layer that is formed on its surface which degrades radiation performance and cannot actively dissipate that is generated when charging or discharging the battery. Furthermore, as the temperature rises, the rate of discharge increases thus drastically shortening the lifespan of the battery.
When the temperature of the battery rises above a reference level due to over-heating, the electrode assembly or the electrolyte decomposes and a large amount of gas is generated. This causes the sheath to swell easily since it comprises a flexible material. The sheath also swells severely due to heat that is supplied from the exterior.
Meanwhile, the battery sheath of conventional lithium polymer batteries use metal foil as the core layer. However, the metal foil is exposed to the exterior along the edge of the battery sheath and is very likely to be short-circuited to a conductor of a protective circuit module or an external set.